Chronic insulin deficiency leads to diminished hypothalamic sensitivity to estrogen. For example, the STZ-induced diabetic rat model, a well known model of chronic insulin deficiency, is characterized by reduced hypothalamic estrogen receptor binding. Moreover, estrogen-facilitated reproductive behavior in STZ-induced diabetic animals is markedly attenuated. The STZ animal model is characterized by both chronic insulin deficiency as well as concomitant hyperglycemia. Previous studies did not examine the relative role of hypoinsulinemia versus hyperglycemia as the causative agent leading to diminished hypothalamic sensitivity to estrogen observed in STZ-induced diabetic animals. Our preliminary data suggest that diabetes-induced reductions in hormone-facilitated reproductive behavior in STZ treated animals re due to hypoinsulinemia rather than hyperglycemia. Therefore, this project proposes to test specific hypotheses about the molecular mechanisms by which insulin maintains hypothalamic responsiveness to estrogen. The major goal of this proposal is to determine a novel direct action of insulin in the hypothalamus. In particular, the proposed research investigates the hypothesis that insulin works in the brain to maintain hypothalamic sensitivity to estrogen and begins to examine insulin's probably mechanisms of action. The potential significance of this work, therefore, is an understanding of the interplay between insulin action and the normal neuroendocrine function of the hypothalamus as well as the consequences of chronic insulin deficiency for hypothalamic responsiveness to estrogen. Specific aim 1 tests the hypothesis that insulin acts directly in the brain to maintain hypothalamic sensitivity to estrogen. Specific experiments will (a) examine whether insulin infusion directly into the third ventricle is able to maintain hypothalamic responsiveness to estrogen; (b) whether euglycemia and/or alternate metabolic fuels are required in addition to insulin to maintain hypothalamic responsiveness to estrogen; and (c) the brain site specificity of insulin action. Specific aim 2 tests the hypothesis that insulin maintains hypothalamic estrogen sensitivity by maintaining hypothalamic estrogen receptor levels. Specific aim 3 tests the hypothesis that the mechanism by which insulin acts in the hypothalamus to maintain hypothalamic estrogen receptor levels is by activating noradrenergic neurotransmission. These studies will advance our understanding of the molecular basis underlying diabetes-induced reductions in hypothalamic sensitivity to estrogen. Because the hypothalamus is a regulatory center for numerous physiologic processes including feeding behavior, thermoregulation, and the control of peripheral glucose levels, the results may also have broader implications for understanding how insulin action affects other facets of hypothalamic function.